Project Summary/Abstract This application seeks funding for a Research Program for acute lymphoblastic leukemia (ALL) in vivo testing as part of the Pediatric Preclinical Testing Consortium (PPTC). Due to the less frequent occurrence of childhood cancers compared with adult cancers only a limited number of pediatric clinical trials can be conducted each year. Therefore, it is essential to select those drugs for pediatric clinical trials that have the maximum likelihood of success. The broad aim of this application is to improve the treatment options for children with aggressive and/or drug resistant ALL by prioritizing new drugs for clinical trials in the disease using state-of-the-art preclinical experimental models. This aim will be accomplished by using a large panel of cell and molecularly defined xenografts that are established in immune-deficient mice to test 6-10 new drugs and/or their combinations annually over a 5 year period, maintaining high technical quality, on schedule, and within the estimated costs. The xenografts to be used in the study grow as orthotopic disease, meaning that the leukemia develops in the same organs in mice as in human patients. All of the xenografts to be used in the study were established from direct patient explants, and were not previously passaged in vitro. Engraftment and responses to treatment will be monitored by measuring the proportion of human leukemia cells in the peripheral blood of mice, which provides a reliable representation of overall leukemia burden in the animals. The broad methodology will use panels of up to 8 xenografts at a time that are inoculated into mice, a period of time to allow the disease to develop, followed by a treatment and monitoring period to assess drug responses. In addition to testing 6-10 new agents/combinations annually, we have also proposed to test 3 novel hypotheses during the course of the funding period. These hypotheses are based on our detailed knowledge of the cell and molecular characteristics of the primary disease and the xenografts, as well as previous evaluation of in vivo xenograft drug responses to new agents. By completing the major objectives outlined in this proposal, in the long term we aim to improve the treatment options and quality of life for children with aggressive forms of ALL who would otherwise succumb to their disease.